Orthopedic implants such as artificial knees and hips are critical to improving the quality of life for millions of people each year. As the population ages, the need for such implants will continue to increase. An important attribute of these devices is how well the body's bone and tissue can bond to them.
One method that is known in the art to promote the attachment of implants to bone is to apply hydroxyapatite to their surface using plasma spray technology. Hydroxyapatite is a natural material to which bones will attach. However, this method is expensive and hydroxyapatite is brittle and difficult to make adhere to the smooth surfaces of implants.
It is also known that open, porous structures can promote the attachment of natural tissue to implanted material. Tantalum is often chosen for such applications because it is extremely corrosion resistant and biocompatible. Porous tantalum can be used as an element in orthopedic devices or they can be made entirely of porous tantalum. This is the subject of U.S. Pat. Nos. 5,282,861; 5,669,909; 5,984,967; 6,645,206; 6,613,091 and 6,375,655. It is well known in the art that porous tantalum can be formed by sintering tantalum powder under the proper conditions. Other methods for producing porous tantalum, such as using chemical vapor deposition to fill a vitreous carbon matrix with tantalum, are also known.
Tantalum, however, is a relatively soft, ductile metal and an implant made entirely of porous tantalum would not be strong enough to be used for a highly stressed part in a hip or knee, for example. In applications requiring mechanical strength, alloys containing cobalt, chromium, nickel, titanium and other materials such as stainless steel are often used. In such cases, it is desirable to create a porous surface layer to help natural tissue to bond. Attaching a porous tantalum layer to such materials requires several steps. This is the subject of U.S. Pat. No. 6,063,442, which describes a method of clamping a porous material to a substrate and using chemical vapor deposition to bond the two. However, in addition to the cost of this method, processing temperatures as high as 925 C are required. These high temperatures can alter the mechanical properties of many alloys. Moreover, clamping a porous layer to the complicated shapes used in orthopedic devices is difficult.
Recently it has been found that small surface features with sizes of approximately 100 nanometers (nm) can promote the attachment of bone cells to metals (Nanobumps Enhance Implants, R&D Magazine, January 2004, p. 46). Surface features of tens to hundreds of nm in size mimic the texture of natural bone and are also comparable to the size of the proteins needed to promote tissue growth. It is believed that the precise shape of these features is not critical to their usefulness and they can be regular or irregular in shape.
Therefore, what is needed is a coating having surface roughness on the order of ten to hundreds of nanometers that can be applied directly to orthopedic implants in a simple manner.